come before a noun, and before an adjective + noun (a tree, a tall tree).

are not used before names of people or places.

Indefinite articles (a / an) --

are used when the nouns are not specific. For example: "A tree is burning." We do not know which tree because it is general, not specific.

are used with singular count nouns. For example: "a cat, an apple."

have two forms: a and an. "A" is used when the first sound of the noun is not a vowel. For example: "a cow, a computer, a book". "An" is used when the first sound of the noun is a vowel (a, e, i, o, u ) sound. For example: "an apple, an orange, an animal". But note that it is "a university" because the first sound is y, even though the first letter is u.

Definite article (the) --

is used when the nouns are specific. For example: "The tree in front of the house is burning." We know which tree (the tree in front of the house). It is specific.

is used with singular and plural count nouns. For example: "the cat, the apples".